In reciprocating engines (reciprocating piston engines), except for some 2-cycle engines, there is an intake valve and an exhaust valve that open and close in synchronization with the rotation of the crankshaft. In this kind of reciprocating engine the movement of the cam shaft, which rotates in synchronization with the rotation of the aforementioned crankshaft (at ½ the rpm in the case of a 4-cycle engine), is transmitted to the aforementioned intake valve and exhaust valve through the rocker arm, to cause the intake valve and exhaust valve to move back-and-forth in the axial direction.
Conventionally, it has been typical for the rocker arm installed in the valve mechanism of this kind of engine to be a molding (cast iron or aluminum die cast article). However, moldings are heavy (in the case of cast iron) or have a large volume in order to maintain sufficient strength (in the case of cast aluminum parts). Moreover, since in most cases the moldings are made using a lost wax process, it is difficult to avoid high manufacturing cost. Therefore, recently, manufacture the aforementioned rocker arm by pressing a sheet metal such as steel sheet has been considered and has been performed in some cases.
The manufacturing method of a this kind of rocker arm made of sheet metal has been disclosed previously, for example as disclosed in Japanese Patent Publication No. Tokukai Hei 3-172506. In the manufacturing method described in this publication, the sheet metal rocker arm is manufactured in one piece by pressing a single sheet of sheet metal. Therefore, the sheet metal rocker arm that is obtained has a nearly uniform thickness over its entire surface.
In contrast to this, a rocker arm that is manufactured by joining or welding two or three members together that are each formed by pressing sheet metal has been known. With this construction, the thickness of each of these members is the same, however, in the case of a rocker arm that is formed by combining a plurality of members in this way, the thickness of the connection section, including the pivot section and the valve engagement, can be made larger than the wall sections.
Of the prior art described above, in the case of Japanese Patent Publication No. Tokukai Hei 3-172506 where the sheet-metal rocker arm is made from one sheet of metal, the thickness of the sheet metal rocker arm is nearly uniform over the entire surface, so the area around the valve engagement that receives strong forces during use, has a large strength disadvantage compared with other parts and rigidity may also become low. When the thickness of the metal sheet used for making the sheet metal rocker arm is increased in order to sufficiently secure the strength and rigidity of the area near the valve engagement, the thickness of the other parts becomes greater than necessary, making it impossible to sufficiently make the sheet-metal rocker arm compact and lightweight, and also increases the cost of material.
On the other hand, in the case of a sheet-metal rocker arm that is made by welding together two or three members that are formed by pressing sheet metal, the thickness of the connection section, including the valve engagement, can be made thicker than other parts, such as the wall sections, however, after the members have been individually made, they must be combined and joined together by welding. Therefore, the number of processing steps increases, and managing the parts can be troublesome. Furthermore, complicated, precision equipment is necessary for positioning the parts when putting them together, so in addition to the increase in the number of processing steps and the need for managing the parts, it is impossible to avoid increased manufacturing cost. Moreover, the quality (precision) of the sheet-metal rocker arm is inferior when compared with a rocker arm made in one piece.
In order to solve the problems mentioned above, an invention was disclosed in Japanese Patent Publication No. Tokukai Hei 11-63515, as shown in FIGS. 1 thru 7, relating to sheet-metal rocker arm and the manufacturing method thereof. As shown in FIG. 1, the sheet-metal rocker arm of this previous invention comprises a pair of wall sections 2 that are nearly parallel with each other, and a first connection section 3 and second connection section 4 that connect one of the edges in the width direction of both of the wall sections 2 together, respectively. Moreover, a pair of concentric circular holes 5 is formed in the middle in the lengthwise direction of both of these wall sections 2, and both ends of a support shaft for supporting the roller that interacts with the cam are supported in these holes 5 such that the roller rotates freely. Of the first connection section 3 and the second connection section 4, an engagement section 6 is formed on one surface of the first connection section 3 for coming into contact with the base of the valve unit, and a second engagement section 7 is formed on the second connection section 4 for coming into contact with the tip end of a rush adjuster.
Of the first engagement section 6 and the second engagement section 7, the first engagement section 6 is formed on its one surface into a concave channel shape by plastic deformation in the thickness direction in the middle in the width direction of the first connection section 3 such that it is depressed more than the other parts of the first connection section 3. Also, there is a raised section 8 on the other surface of the first connection section 3, having a trapezoidal cross-section that is protruded outward in a banked shape as the first engagement section 6 is formed. On the other hand, the second connection section 7 is formed into a spherical concave shape by plastic deformation in the thickness direction in the center of the second connection section 4.
When making the sheet-metal rocker arm as described above, first, in a first process, a first blank 9 is made as shown in FIG. 2. In other words, in this first process, a metal sheet (flat sheet or coiled sheet) such as a carbon steel sheet with a thickness of 3 to 4 mm and having sufficient rigidity is supplied between the punching die and cradle die of a press apparatus (not shown in the figure), and the first blank 9 is punch-pressed and formed between these two dies.
As shown in FIG. 2(A), this first blank 9 has a nearly diamond shape with rounded corners and with one end in the lengthwise direction (right end in FIG. 2(A)) cut off, and has a thickness t9 (FIG. 2(B)). In the center in the width direction (up and down directions in FIG. 2(A)) of this first blank 9, in the section slightly inside of the two dot-dashed lines α shown in FIG. 2(A) (the center in the width direction), the portion with width W10 is a base section 10 that is continuous in lengthwise direction (left and right directions in FIG. 2(A)) of the first blank 9. On both sides in the width direction of this base section 10 there is a pair of nearly triangular wing-shaped sections 11.
Next, in a second process, a through hole 12 is formed in the center of the first blank 9 as shown in FIG. 3(A), to become the second blank 13. The shape of this through hole 12 is formed in nearly an hourglass shape with the center section in the lengthwise direction of both edges in the width direction formed with a pair of tongue-shaped sections 14 in a partial arc shape protruding toward each other. These two tongue-shaped sections 14 are provided for forming circular holes 5 (see FIG. 1 and FIG. 7) for supporting both ends of a support shaft for supporting a roller, to be described later, such that it rotates freely. In addition, semi-circular cutout sections 15 are formed in each of the four corners of the through hole 12. These cutout sections 15 are provided for making it easier in the third process to bend the base section 10 in an arc shape in cross section to form a curved section 16 (see FIG. 4).
This second blank 13 described above is formed by supplying the first blank 9 between the punching die and cradle die of a press apparatus (not shown) and punching out the through hole 12 between these two dies. The width W10 of the base section 10 of the first blank 9 and second blank 13 is wider than the width W17 (W10>W17) of a first intermediate blank 17 (see FIG. 4) which is the distance between the outer sides of a pair of wall sections 2 that are formed ill the third process described next. As the width W10 of the base section 10 is made wider than the width W17 of first intermediate blank 17, the distance D14 between the pair of tongue-shaped sections 14 is also made large.
When the distance D14 between the pair of tongue-shaped sections 14 is made large in this way, it is possible to secure the life of the punching die used for punching out the aforementioned through hole 12. In other words, when the width of the center section of the through hole 12 is narrow, the load on the punching die used for punching out this through hole is large, which shortens the life of the punching die. On the other hand, when the width of the center section of the through hole 12, that is the distance D14 between the pair of tongue-shaped sections 14, is large, the load on the punching die used for forming the through hole 12 is decreased, making it possible to secure the durability of the punching die, and thus making it possible to reduce costs.
As to the order of forming the second blank 13, it is also possible to first, form the through hole 12 instead of by the second process described above, then to form the base section 10 and wing-shaped section 11 instead of by the first process. Furthermore, if the capacity of the press apparatus is sufficient and the punching die and cradle die are capable, it is possible to directly form the second blank 13, as shown in FIG. 3, from the sheet metal material.
In either case, the second blank 13 that is formed in the shape shown in FIG. 3 is then formed into the first intermediate blank 17, as shown in FIG. 4, by the following third process. In the third process, the second blank 13 is supplied between the pressing die and cradle die of a press apparatus (not shown in the figure), and strongly pressed to bend the base section 10 and wing-shaped sections 11 of the second blank 13. This second blank 13 is formed into a first intermediate blank 17, having a pair of wall sections 2 on the left and right in the width direction, and a curved section 16 for connecting the edges in the width direction (left and right directions in FIGS. 4(C) and 4(D)) of these wall sections 2. This curved section 16 is formed in a semi-cylindrical shape such that it is discontinuous in a portion that corresponds to the through hole 12 in the center in the lengthwise direction (left and right directions in FIG. 4(A)) of the first intermediate blank 17. Of the curved section 16 that is divided into two by the through hole 12 in this way, the portion on one end side (right end side in FIG. 4(A)) becomes the engagement section 6 that comes in contact with the base of the valve knit, and the portion on other end side (left end side in FIGS. 4(A) and 4(B)) becomes the second engagement section 7 (see FIG. 1, FIG. 6 and FIG. 7) that comes in contact with the tip end of a rush adjuster.
As described above, the width W17 of the first intermediate blank 17, which is the distance between the outside surfaces of the pair of wall sections 2, is smaller than the width W10 of the base section 10 of the first and second blanks 9 and 13. In other words, the curved section 16 in the first intermediate blank 17, which acts as the connection section for connecting the edges in the width direction of the pair of wall sections 2, is formed in a semi-cylindrical shape as shown in FIGS. 4(C) and 4(D). Since the width of this semi-cylindrical curved section 16 is less than the width W10 of the flat base section 10 which is to be formed into the semi-cylindrical carved section 16, it is possible to make the width W10 of this base section 10 larger than the width W17 of the first intermediate blank 17, that is the distance between the pair of left and right wall sections 2, 2 (W10>W17), and thus it is possible to make the distance D14 between the pair of tongue-shaped sections 14 large. The thickness t16 of the curved section 16 of the first intermediate blank 17 shown in FIG. 4, which is obtained from the third process described above, is nearly the same as the thickness t9 of the first blank 9 (t16≈9).
Next, in a fourth process, pressing is performed for at least one end of the curved section 16 that forms the engagement section 6 that comes in contact with the base end of the valve unit, in order to increase the thickness. In this case, in order to obtain the desired thickness after the pressing process, it is necessary to regulate the shape and dimensions of the curved section 16. In other words, selecting the size and dimensions of the curved section 16 determines the aforementioned thickness in the pressing process. Moreover, at the same time that the curved section 16 is formed on the first intermediate blank 17, the pair of left and right wall side sections 2 is also formed. In other words, as the curved section 16 is formed, the wing-shaped sections 11, which are formed on both ends in the width direction of the first and second blanks 9 and 13, and the tongue-shaped sections 14, which are formed on the inner edges in the through-hole 12 in the center of the second blank 13, are raised upright such that they become the pair of nearly parallel side wall sections 2.
In the fourth process, the curved section 16 of the first intermediate blank 17, which is formed as described above, is pressed to form the second intermediate blank 18 as shown in FIG. 5. In other words, in the fourth process, the curved section 16 is flattened and its thickness is increased to form a connection section 3 and second connection 4 having thick nesses t3 and t4, which are greater than the thickness t9 (see FIG. 2B) of the first blank (t9<t3, t4). The curved section 16 does not need to be a semi-circular cylindrical shape, but can be curved in a semi-oval or semi-elliptical cylindrical shape etc.
The fourth process mentioned above is performed by setting the curved section 16 of the first intermediate blank 17 between a pressing die for pressing and a cradle die and performing cold forging by pressing to plastically deform the curved section 16. As a result, a flat connection section 3 and second connection 4 are formed. When plastically deforming the curved section 16 to form the connection section 3 and second connection section 4, as the curved section 16 with arc-shaped cross-section is deformed to become the flat connection section 3 and second connection section 4, they are thickened to the thickness t3 and t4. In this way, the process where the thickness is increased at the same time as the curved section 16 with arc-shaped cross-section is deformed to become the flat connection section 3 and second connection section 4, can be easily performed by pressing with a press.
In the example shown in the figures, the thickness of connection section 3 formed on one side is increased as well as the thickness of the second connection section 4 that is formed on the other side is increased. However, when the sheet-metal rocker arm is in operation, an especially large stress is applied to the connection section 3 provided with the engagement section 6 that comes in contact with the base end of the valve unit. Therefore, the thickness of the second connection section 4 on the other side must not necessarily increased. When it is not necessary to increase the thickness, the curved section 16 can be plastically deformed to simply form a flat connection section. However, by making the thickness of the connection section 3 and second connection section 4 the same less processing is required, which is advantageous from the aspect of cost.
In the fourth process, when the connection section 3 and second connection section 4 with a relatively large thickness are formed in the first intermediate blank 17 to make a second intermediate blank 18, then in the fifth process a plastic deformation process or cutting process, and when necessary a grinding process, is performed for the connection section 3 and second connection section 4. In other words, as shown in FIG. 6, the engagement section 6, which comes in contact with the base end of the valve unit (not shown in the figure), is formed on the connection section 3. Moreover, the second engagement section 7, which comes in contact with the tip end of a rush adjuster, is formed on the second connection section 4. In this fifth process, the connection section 3 of the second intermediate blank 18 is set between the pressing die and cradle die of a forging apparatus to perform cold forging of the connection section 3 to form a concave engagement section 6 with a bottom surface that is curved in a convex shape as shown in FIGS. 6(A), 6(B) and 6(D). Moreover, the second connection section 4 is set between the pressing die and cradle die of a different forging apparatus (not shown in the figure) to perform cold forging of this second connection section 4 to form a spherical concave hole or second engagement section 7, as shown in FIG. 6(A), 6(B) and 6(C). From this fifth process, an engagement section 6 and second engagement section 7 are formed on the respective connection section 3 and second connection section 4, which have thicknesses that are greater than the thickness of the first blank 9, to form the third intermediate blank 19. The order of the processes from the first to fifth can be changed. For example, it is possible to change the order of the processes above or the shape of the intermediate blanks such that they are suitable for transfer-press processing or progressive processing. However, in the end, the third intermediate blank 19 should be obtained.
The third intermediate blank 19 that is obtained in this way is processed in the sixth process by pressing or drilling to form circular holes 5 at matching positions in the middle of the pair of side wall sections 2 to form the completed sheet-metal rocker arm 1 as shown in FIG. 1 and FIG. 7. As described above, both of these circular holes 5 are provided for supporting both of the ends of a support shaft that rotatably supports the roller. In other words, the roller is supported in the middle of the support shaft whose ends are supported in the aforementioned circular holes 5 such that the roller rotates freely, and the outer peripheral surface of the roller comes in contact with the outer peripheral surface of the cam to transform the rotating motion of the cam shaft to rocking motion of the sheet-metal rocker arm 1.
The sheet-metal rocker arm and the manufacturing method for it of the previous invention described above, not only makes it possible to improve the strength and rigidity of the rocker arm, but by reducing the number of processes and parts, also make it possible to reduce cost, improve precision and simplify the equipment used.
However, in order for it to obtain more strength so as to be possible to install the rocker arm in an engine with large output, improvements of the following aspect related to the engagement section 6 that comes in contact with the base of the valve unit are desired. In this case, it is difficult, with the manufacturing method for the sheet-metal rocker arm of the previous invention described above, to improve this aspect.
This aspect will be explained using FIG. 8, which shows the cross-section shape of the engagement section 6 that is formed on the sheet-metal rocker arm 1 of the previous invention described above. By deforming the middle of the connection section 3 in the thickness direction, the aforementioned engagement section 6 is formed such that one surface (bottom surface in FIG. 8) of the connection section 3 is more concave than other sections of the connection section 3, and there is a bulge section 8 with a trapezoidal-shaped cross-section that protrudes in the shape of an embankment on the other surface (top surface in FIG. 8) of the connection section 3. In the case of the conventional construction, the width W20 of the center of this bulge section 8, which corresponds to the top of the trapezoidal shape in cross section of this bulge section 8, was the same as or greater than the width W6 of the aforementioned engagement section 6 (W20≧W6). In addition, both edges in the width direction (left and right direction in FIG. 8) of the center section 20 are in nearly the same position as or outside of in the width direction of both edges in the width direction of the engagement section 6.
The aforementioned engagement section 6 and bulge section 8 are formed by pressing tightly a portion, corresponding to the connection section 3 on the end of the second intermediate blank 18, in the fifth process, between the pressing die and cradle die of a press apparatus. At that time, when the width W20 of the center section 20 of the bulge section 8 is the same as or greater than the width W6 of the engagement section 6, a shear force is applied to part in the width direction of the connection section 3 at both ends in the width direction of the engagement section 6 (section shown by the dot-dash line β in FIG. 8). As a result, internal distortion occurs in this section, and not only does it become easy for cracking or the like to occur during manufacture, but there is also a possibility that damage such as cracking could occur at both ends in the width direction of the engagement section 6.
The force applied to the engagement section 6 during operation becomes large as the spring force of the return spring to energize the valve unit whose base section comes in contact with the engagement section 6 is made large in order that the output of the engine becomes large. Also, in order to be able to install the sheet-metal rocker arm in a high-output engine and to secure sufficient durability, it is desired that the strength of the engagement sections 6 be increased.
In order to accomplish that, it is also desired that the thickness of the engagement section 6 be increased, however in the manufacturing method of the previous invention described above, the amount that the thickness can be increased is limited to about 5 to 40% of the thickness of the raw sheet. For example, it is difficult to increase the thickness t3 of the connection section 3 of the engagement section 6 to nearly two times or more than two times the thickness t, of the pair of side wall sections 2.
In consideration of the problems described above, an objective of this invention is to provide a sheet-metal rocker arm and manufacturing method that solves these problems.